Concurrently using two or more laser beams to cut a workpiece, such as lumber or wood, is known. For example, U.S. Pat. No. 4,943,700 to Hughes et al. suggests applying a first laser beam "pre-pulse" of a few nanoseconds to a surface of a log to generate a line plasma. Subsequently, a high powered laser beam is directed upon the plasma, self-focusing the laser beam along a plane and producing a hairline cut through the log. It is suggested that the log be cut from opposing sides.
Another example is U.S. Pat. No. 4,689,467 to Inoue which discloses a laser machining apparatus. An auxiliary energy source, such as a secondary laser or plasma generator, is used to condition or preheat a material which is subsequently cut by a powerful primary laser beam. If a workpiece is sufficiently thin, the secondary heating source can be applied to the side opposite the primary laser beam. However, only one laser beam is used to cut or machine a workpiece.
Further, U.S. Pat. No. 4,625,093 to Chryssolouris shows a combination of two laser beams cutting a workpiece. The laser beams are directed along non-coaxial axes to produce intersecting kerfs. The workpiece is moved relative to the laser beams to produce a separate solid removable portion from the workpiece.
However, these systems of cutting wood or workpieces have drawbacks. The high-powered self-focusing laser beam of the '700 patent produces only a very narrow hairline cut with the high powered cutting laser beam remaining substantially non-divergent. While this allegedly allows deep penetration through relatively thick workpieces, the laser beam exiting from the cut workpiece remains focused and powerful. Therefore, the concentrated beam is capable of damaging other downstream structures such as an opposing laser or support structure. Further, the high-powered laser beam is sufficiently powerful that it is recommended that the cutting operation be performed within an evacuated safety enclosure to protect against laser beam ionization of the air and to protect personnel. Moreover, this system is complex in that it requires a first laser beam which creates a plasma upon which a second laser beam is directed so as to be self-focusing. Also, with a laser beam as powerful as the one suggested in the '700 patent, substantial efficiency losses occur due to the high power of lasers involved. Finally, difficulties in over-heating lenses often occur when laser beams of the suggested high power are focussed through such lenses.
The laser cutting system of the '467 patent teaches cutting a workpiece from only a single side. Hence, this system is relatively slow in cutting a workpiece. Also, the system works poorly where the material to be cut has a low thermal conductivity, such as wood.
The laser cutting of the '093 patent uses intersecting, non-coincident laser beams for cutting or slicing thin sections from a large workpiece. This system does not use beams which are coincident to cut through a workpiece from both sides to produce a through-thickness rip and/or cross-cut.
The present invention is intended to overcome the shortcomings of the laser cutting systems described in the above patents.